Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch

Avi Kivity
2010-07-05T09:20:56Z

On 07/05/2010 12:09 PM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>
>
>> I'm not convinced we can bypass the checks. Consider:
>>
>>
>> VCPU0 VCPU1
>>
>> #PF
>> walk_addr
>> -> gpml4e0,gpdpe0,gpde0,gpte0
>>
>> replace gpdpe0 with gpdpe1
>> #PF
>> walk_addr
>> -> gpml4e0,gpdpe1,gpde1,gpte1
>> fetch
>> -> establish hpml4e0,hpdpte1,hpde0,hpte1
>> fetch
>> read hpdpe1
>> if (present(hpdpe1))
>> continue;
>> ...
>> write hpte0 using shadow hieratchy for hpte1
>>
>>
> Ah, i missed this case, thanks for you point it out, i'll fix it in
> the next version.
>

Note: I think we have to check _after_ kvm_mmu_get_page(), otherwise we
might be checking a page that is not write-protected and can change again.

So the logic needs to be something like

for_each_shadow_entry:
if (!last_level && !present(*spte))
kvm_mmu_get_page
verify gpte
if (last_level)
mmu_set_spte()

--
error compiling committee.c: too many arguments to function